As shown schematically in FIG. 1 (cross-sectional view) and FIG. 2 (longitudinal-sectional view), methods are known in which a workpiece 4 is displaced by rotation along one rotational direction 6, and then is reshaped by means of a cylinder or roller 2 acting on the workpiece from an external position and mounted within a bearing housing 1 at relative displacement from the workpiece 4 along an axial-displacement direction 5 with respect to the reshaping roller 2. In most cases, this reshaping occurs in a manner such that the outer reshaping rollers 2 press the material of the workpiece 4 in the area to be reshaped against a mandrel 3 so that the material is partially displaced axially, radially, and tangentially in a fluid condition, and this leads to a reduction in wall thickness of the workpiece 4, whereby the reduced wall thickness to be achieved results from the separation between the reshaping roller 2 and the mandrel 3.
For this, the potential reduction in wall thickness of the workpiece 4 is limited by the value of wall thickness, the stiffness of the material, the friction between the inner walls of the blank mold 4 and the mandrel 3, and the potential number of reshaping rollers 2 at the circumference of the workpiece 4.
The causes for this limitation is:
The magnitude of force that may be partially introduced into the workpiece by means of the reshaping roller 2, generating a yield stress there;
The stiffness of the material, which cannot be influenced by the reshaping process;
The magnitude of friction between the workpiece 4 and the mandrel 3, which is dictated by the type of process;
The number of reshaping rollers 2 that may be mounted about the circumference of the workpiece 4; and
The dimensions of the mounting of the reshaping rollers 2, which in turn are determined by the service life of the bearing and its dimensions, as well as the size of the reshaping rollers 2.
Also, the effect of the contact-pressure force of the reshaping rollers 2 is reduced as the wall thickness to be reshaped increases, so that it is no longer possible after a certain wall thickness to bring the material in the pressure areas 7 (effective area) of the reshaping rollers 2 into a fluid condition. Thus, the thickness of the walls to be reduced using known pressure-roller processes is limited by the lacking yield stress of the material.